The increment value when calculating the new timer time
(wait_for). Note that this value can be negative
and that a timer restart can therefor lead to a wait_for
value of zero! It is up to the user to be aware of the
consequences of a wait_for value of zero.

max_retries = infinity | infinity_restartable | integer() >= 0

The maximum number of repetitions of the timer.
There is a special case for this field. When the
max_retries has the value infinity_restartable,
it means that the timer is restartable as long as some
external event occurs (e.g. receipt of a pending
message for instance). But the timer will never be
restarted "by itself", i.e. when the timer expires
(whatever the timeout time), so does the timer.
Whenever the timer is restarted, the timeout time will
be calculated in the usual way! Also, as mentioned
above, beware the consequences of setting the value to
infinity if incr has been set to an
negative value.

EXPORTS

Users may either explicitly be registered with
megaco:start_user/2 and/or be statically configured by
setting the application environment variable 'users' to a
list of {UserMid, Config} tuples. See the function
megaco:start_user/2 for details.

Lists all active connections for this user. Returns a
list of megaco_conn_handle records.

receive_handle

Construct a megaco_receive_handle record from user config

trans_id

Current transaction id.
A positive integer or the atom
undefined_serial (in case no messages has been sent).

min_trans_id

First trans id.
A positive integer, defaults to 1.

max_trans_id

Last trans id.
A positive integer or infinity,
defaults to infinity.

request_timer

Wait for reply.
The timer is cancelled when a reply is received.
When a pending message is received, the timer is
cancelled and the long_request_timer is started instead
(see below). No resends will be performed from this point
(since we now know that the other side has received the
request).
When the timer reaches an intermediate expire, the request
is resent and the timer is restarted.
When the timer reaches the final expire, either the function
megaco:call will return with {error, timeout}
or the callback function handle_trans_reply will be
called with UserReply = {error, timeout} (if
megaco:cast was used).
A Megaco Timer (see explanation above),
defaults to #megaco_incr_timer{}.

long_request_timer

Wait for reply after having received a pending message.
When the timer reaches an intermediate expire, the timer
is restarted.
When a pending message is received, and the
long_request_timer
is not "on it's final leg", the timer will be
restarted, and, if long_request_resend = true, the
request will be re-sent.
A Megaco Timer (see explanation above),
defaults to 60 seconds.

long_request_resend

This option indicates weather the request should be
resent until the reply is received,
even though a pending message has been received.
Normally, after a pending message has been received,
the request is not resent
(since a pending message is an indication that the
request has been received). But since the reply (to the
request) can be lost, this behaviour has it's values.
It is ofcourse pointless to set this value to true
unless the long_request_timer (see above) is also set
to an incremental timer (#megaco_incr_timer{}).
A boolean,
defaults to false.

reply_timer

Wait for an ack.
When a request is received, some info
related to the reply is store internally (e.g. the
binary of the reply). This info will live until either
an ack is received or this timer expires. For instance,
if the same request is received again (e.g. a request
with the same transaction id), the (stored) reply will
be (re-) sent automatically by megaco.
If the timer is of type #megaco_incr_timer{},
then for each intermediate timout, the reply will be resent
(this is valid until the ack is received or
the timer expires).
A Megaco Timer (see explanation above), defaults to 30000.

auto_ack

Automatic send transaction ack when the transaction
reply has been received (see trans_ack below).
This is used for three-way-handshake.
A boolean, defaults to false.

trans_ack

Shall ack's be accumulated or not.
This property is only valid if auto_ack is true.
If auto_ack is true, then if trans_ack is
false, ack's will be sent immediatelly.
If trans_ack is true, then
ack's will instead be sent to the transaction
sender process for accumulation and later sending
(see trans_ack_maxcount, trans_req_maxcount,
trans_req_maxsize, trans_ack_maxcount and
trans_timer).
See also transaction sender for more info.
An boolean, defaults to false.

trans_ack_maxcount

Maximum number of accumulated ack's. At most this many ack's
will be accumulated by the transaction sender (if started and
configured to accumulate ack's).
See also transaction sender for more info.
An integer, defaults to 10.

trans_req

Shall requests be accumulated or not.
If trans_req is false, then request(s)
will be sent immediatelly (in it's own message).
If trans_req is true, then request(s) will
instead be sent to the transaction sender process for
accumulation and later sending
(see trans_ack_maxcount, trans_req_maxcount,
trans_req_maxsize, trans_ack_maxcount and
trans_timer).
See also transaction sender for more info.
An boolean, defaults to false.

trans_req_maxcount

Maximum number of accumulated requests. At most this many
requests will be accumulated by the transaction sender
(if started and configured to accumulate requests).
See also transaction sender for more info.
An integer, defaults to 10.

trans_req_maxsize

Maximum size of the accumulated requests. At most this much
requests will be accumulated by the transaction sender
(if started and configured to accumulate requests).
See also transaction sender for more info.
An integer, defaults to 2048.

trans_timer

Transaction sender timeout time. Has two functions. First, if
the value is 0, then transactions will not be accumulated
(e.g. the transaction sender process will not be started).
Second, if the value is greater then 0 and auto_ack
and trans_ack are both true or
if trans_req is true,
then transaction sender will be started and transactions
(which is depending on the values of auto_ack,
trans_ack and trans_req) will be accumulated,
for later sending.
See also transaction sender for more info.
An integer, defaults to 0.

pending_timer

Automatically send pending if the timer expires before a
transaction reply has been sent. This timer is also called
provisional response timer.
A Megaco Timer (see explanation above), defaults to 30000.

sent_pending_limit

Sent pending limit (see the MGOriginatedPendingLimit
and the MGCOriginatedPendingLimit of the megaco root package).
This parameter specifies how many pending messages that can
be sent (for a given received transaction request).
When the limit is exceeded, the transaction is aborted
(see handle_trans_request_abort) and an error message
is sent to the other side.
Note that this has no effect on the actual sending of
pending transactions. This is either implicit (e.g. when
receiving a re-sent transaction request for a request which
is beeing processed) or controlled by the pending_timer,
see above.
A positive integer or infinity,
defaults to infinity.

recv_pending_limit

Receive pending limit (see the MGOriginatedPendingLimit
and the MGCOriginatedPendingLimit of the megaco root package).
This parameter specifies how many pending messages that can
be received (for a sent transaction request).
When the limit is exceeded, the transaction is considered
lost, and an error returned to the user (through the call-back
function handle_trans_reply).
A positive integer or infinity,
defaults to infinity.

send_mod

Send callback module which exports send_message/2. The
function SendMod:send_message(SendHandle, Binary) is
invoked when the bytes needs to be transmitted to the
remote user.
An atom, defaults to megaco_tcp.

encoding_mod

Encoding callback module which exports encode_message/2
and decode_message/2. The function
EncodingMod:encode_message(EncodingConfig,
MegacoMessage) is invoked whenever a 'MegacoMessage'
record needs to be translated into an Erlang binary. The
function EncodingMod:decode_message(EncodingConfig,
Binary) is invoked whenever an Erlang binary needs to be
translated into a 'MegacoMessage' record.
An atom, defaults to megaco_pretty_text_encoder.

encoding_config

Encoding module config.
A list, defaults to [].

protocol_version

Actual protocol version.
An integer, default is 1.

strict_version

Strict version control, i.e. when a message is received,
verify that the version is that which was negotiated.
An boolean, default is true.

reply_data

Default reply data.
Any term, defaults to the atom undefined.

user_mod

Name of the user callback module. See the the reference
manual for megaco_user for more info.

user_args

List of extra arguments to the user callback
functions. See the the reference manual for megaco_user
for more info.

threaded

If a received message contains several transaction requests,
this option indicates whether the requests should be handled
sequencially in the same process (false), or if each
request should be handled by it's own process (true
i.e. a separate process is spawned for each request).
An boolean, defaults to false.

resend_indication

This option indicates weather the transport module
should be told if a message send is a resend or not.
If false, megaco messages are sent using the
send_message
function.
If true, megaco message re-sends are made using the
resend_message
function. The initial message send is still done using the
send_message
function.
The special value flag instead indicates that the
function
send_message/3
shall be used.
A resend_indication(),
defaults to false.

segment_reply_ind

This option specifies if the user shall be notified of received
segment replies or not.
See
handle_segment_reply
callback function for more information.
A boolean,
defaults to false.

segment_recv_timer

This timer is started when the segment indicated by the
segmentation complete token is received, but all
segments has not yet been received.
When the timer finally expires, a "megaco segments not
received" (459) error message is sent to the other side
and the user is notified with a segment timeoutUserReply in either the
handle_trans_reply callback function or
the return value of the
call function.
A Megaco Timer (see explanation above),
defaults to 10000.

segment_send

Shall outgoing messages be segmented or not:

none

Do not segment outgoing reply messages. This is usefull when
either it is known that messages are never to large or
that the transport protocol can handle such things
on it's own (e.g. TCP or SCTP).

integer() > 0

Outgoing reply messages will be segmented as needed
(see max_pdu_size below). This value, K, indicate
the outstanding window, i.e. how many segments can be
outstanding (not acknowledged) at any given time.

infinity

Outgoing reply messages will be segmented as needed
(see max_pdu_size below). Segment messages
are sent all at once (i.e. no acknowledgement awaited
before sending the next segment).

Defaults to none.

max_pdu_size

Max message size. If the encoded message (PDU) exceeds
this size, the message should be segmented, and then
encoded.
A positive integer or infinity,
deefaults to infinity.

Opaque send handle whose contents is internal for the
send module. May be any term.

local_mid

The local mid (of the connection, i.e. the own mid).
megaco_mid().

remote_mid

The remote mid (of the connection).
megaco_mid().

receive_handle

Construct a megaco_receive_handle record.

trans_id

Next transaction id. A positive integer or the atom
undefined_serial (only in case of error).
Note that transaction id's are (currently) maintained
on a per user basis so there is no way to be sure that
the value returned will actually be used for a transaction
sent on this connection (in case a user has several
connections, which is not at all unlikely).

max_trans_id

Last trans id.
A positive integer or infinity,
defaults to infinity.

request_timer

Wait for reply.
The timer is cancelled when a reply is received.
When a pending message is received, the timer is
cancelled and the long_request_timer is started instead
(see below). No resends will be performed from this point
(since we now know that the other side has received the
request).
When the timer reaches an intermediate expire, the request
is resent and the timer is restarted.
When the timer reaches the final expire, either the function
megaco:call will return with {error, timeout}
or the callback function handle_trans_reply will be
called with UserReply = {error, timeout} (if
megaco:cast was used).
A Megaco Timer (see explanation above),
defaults to #megaco_incr_timer{}.

long_request_timer

Wait for reply after having received a pending message.
When the timer reaches an intermediate expire, the timer
restarted.
When a pending message is received, and the
long_request_timer
is not "on it's final leg", the timer will be
restarted, and, if long_request_resend = true, the
request will be re-sent.
A Megaco Timer (see explanation above),
defaults to 60 seconds.

long_request_resend

This option indicates weather the request should be
resent until the reply is received,
even though a pending message has been received.
Normally, after a pending message has been received,
the request is not resent
(since a pending message is an indication that the
request has been received). But since the reply (to the
request) can be
lost, this behaviour has it's values.
It is ofcourse pointless to set this value to true
unless the long_request_timer (see above) is also set
to an incremental timer (#megaco_incr_timer{}).
A boolean,
defaults to false.

reply_timer

Wait for an ack.
When a request is received, some info
related to the reply is store internally (e.g. the
binary of the reply). This info will live until either
an ack is received or this timer expires. For instance,
if the same request is received again (e.g. a request
with the same transaction id), the (stored) reply will
be (re-) sent automatically by megaco.
If the timer is of type #megaco_incr_timer{},
then for each intermediate timout, the reply will be resent
(this is valid until the ack is received or
the timer expires).
A Megaco Timer (see explanation above), defaults to 30000.

auto_ack

Automatic send transaction ack when the transaction
reply has been received (see trans_ack below).
This is used for three-way-handshake.
A boolean, defaults to false.

trans_ack

Shall ack's be accumulated or not.
This property is only valid if auto_ack is true.
If auto_ack is true, then if trans_ack is
false, ack's will be sent immediatelly.
If trans_ack is
true, then ack's will instead be sent to the transaction
sender process for accumulation and later sending
(see trans_ack_maxcount, trans_req_maxcount,
trans_req_maxsize, trans_ack_maxcount and
trans_timer).
See also transaction sender for more info.
An boolean, defaults to false.

trans_ack_maxcount

Maximum number of accumulated ack's. At most this many ack's
will be accumulated by the transaction sender (if started and
configured to accumulate ack's).
See also transaction sender for more info.
An integer, defaults to 10.

trans_req

Shall requests be accumulated or not.
If trans_req is false, then request(s)
will be sent immediatelly (in it's own message).
If trans_req is true, then request(s) will
instead be sent to the transaction sender process for
accumulation and later sending
(see trans_ack_maxcount, trans_req_maxcount,
trans_req_maxsize, trans_ack_maxcount and
trans_timer).
See also transaction sender for more info.
An boolean, defaults to false.

trans_req_maxcount

Maximum number of accumulated requests. At most this many
requests will be accumulated by the transaction sender
(if started and configured to accumulate requests).
See also transaction sender for more info.
An integer, defaults to 10.

trans_req_maxsize

Maximum size of the accumulated requests. At most this much
requests will be accumulated by the transaction sender
(if started and configured to accumulate requests).
See also transaction sender for more info.
An integer, defaults to 2048.

trans_timer

Transaction sender timeout time. Has two functions. First, if
the value is 0, then transactions will not be accumulated
(e.g. the transaction sender process will not be started).
Second, if the value is greater then 0 and auto_ack
and trans_ack is true or if trans_req is true,
then transaction sender will be started and transactions
(which is depending on the values of auto_ack,
trans_ack and trans_req) will be accumulated,
for later sending.
See also transaction sender for more info.
An integer, defaults to 0.

pending_timer

Automatic send transaction pending if the timer expires
before a transaction reply has been sent. This timer is
also called provisional response timer.
A Megaco Timer (see explanation above), defaults to 30000.

sent_pending_limit

Sent pending limit (see the MGOriginatedPendingLimit
and the MGCOriginatedPendingLimit of the megaco root package).
This parameter specifies how many pending messages that can
be sent (for a given received transaction request).
When the limit is exceeded, the transaction is aborted
(see handle_trans_request_abort) and an error message
is sent to the other side.
Note that this has no effect on the actual sending of
pending transactions. This is either implicit (e.g. when
receiving a re-sent transaction request for a request which
is beeing processed) or controlled by the pending_timer,
see above.
A positive integer or infinity,
defaults to infinity.

recv_pending_limit

Receive pending limit (see the MGOriginatedPendingLimit
and the MGCOriginatedPendingLimit of the megaco root package).
This parameter specifies how many pending messages that can
be received (for a sent transaction request).
When the limit is exceeded, the transaction is considered
lost, and an error returned to the user (through the call-back
function handle_trans_reply).
A positive integer or infinity,
defaults to infinity.

send_mod

Send callback module which exports send_message/2. The
function SendMod:send_message(SendHandle, Binary) is
invoked when the bytes needs to be transmitted to
the remote user.
An atom, defaults to megaco_tcp.

encoding_mod

Encoding callback module which exports encode_message/2
and decode_message/2. The function
EncodingMod:encode_message(EncodingConfig, MegacoMessage)
is invoked whenever a 'MegacoMessage' record needs to be
translated into an Erlang binary. The function
EncodingMod:decode_message(EncodingConfig, Binary) is
invoked whenever an Erlang binary needs to be translated
into a 'MegacoMessage' record.
An atom,
defaults to megaco_pretty_text_encoder.

encoding_config

Encoding module config.
A list, defaults to [].

protocol_version

Actual protocol version.
An positive integer, Current default is 1.

strict_version

Strict version control, i.e. when a message is received,
verify that the version is that which was negotiated.
An boolean, default is true.

reply_data

Default reply data.
Any term, defaults to the atom undefined.

threaded

If a received message contains several transaction requests,
this option indicates whether the requests should be handled
sequencially in the same process (false), or if each
request should be handled by it's own process (true
i.e. a separate process is spawned for each request).
An boolean, defaults to false.

resend_indication

This option indicates weather the transport module
should be told if a message send is a resend or not.
If false, megaco messages are sent using the
send_message/2
function.
If true, megaco message re-sends are made using the
resend_message
function. The initial message send is still done using the
send_message
function.
The special value flag instead indicates that the
function
send_message/3
shall be used.
A resend_indication(),
defaults to false.

segment_reply_ind

This option specifies if the user shall be notified of received
segment replies or not.
See
handle_segment_reply
callback function for more information.
A boolean,
defaults to false.

segment_recv_timer

This timer is started when the segment indicated by the
segmentation complete token (e.g. the last of the segment
which makes up the reply) is received, but all
segments has not yet been received.
When the timer finally expires, a "megaco segments not
received" (459) error message is sent to the other side
and the user is notified with a
segment timeoutUserReply in either the
handle_trans_reply
callback function or
the return value of the
call function.
A Megaco Timer (see explanation above),
defaults to 10000.

segment_send

Shall outgoing messages be segmented or not:

none

Do not segment outgoing reply messages. This is usefull when
either it is known that messages are never to large or
that the transport protocol can handle such things
on it's own (e.g. TCP or SCTP).

integer() > 0

Outgoing reply messages will be segmented as needed
(see max_pdu_size below). This value, K, indicate
the outstanding window, i.e. how many segments can be
outstanding (not acknowledged) at any given time.

infinity

Outgoing reply messages will be segmented as needed
(see max_pdu_size below). Segment messages
are sent all at once (i.e. no acknowledgement awaited
before sending the next segment).

Defaults to none.

max_pdu_size

Max message size. If the encoded message (PDU) exceeds
this size, the message should be segmented, and then
encoded.
A positive integer or infinity,
deefaults to infinity.

Activates a connection to a remote user. When this is done
the connection can be used to send messages (with
SendMod:send_message/2). The ControlPid is the identifier
of a process that controls the connection. That process will
be supervised and if it dies, this will be detected and the
UserMod:handle_disconnect/2 callback function will be
invoked. See the megaco_user module for more info about the
callback arguments. The connection may also explicitly be
deactivated by invoking megaco:disconnect/2.

The ControlPid may be the identity of a process residing on
another Erlang node. This is useful when you want to
distribute a user over several Erlang nodes. In such a case
one of the nodes has the physical connection. When a user
residing on one of the other nodes needs to send a request
(with megaco:call/3 or megaco:cast/3), the message will
encoded on the originating Erlang node, and then be
forwarded to the node with the physical connection. When the
reply arrives, it will be forwarded back to the originator.
The distributed connection may explicitely be deactivated by
a local call to megaco:disconnect/2 or implicitely when
the physical connection is deactivated (with megaco:disconnect/2,
killing the controlling process, halting the other node, ...).

The call of this function will trigger the callback
function UserMod:handle_connect/2 to be invoked. See the
megaco_user module for more info about the callback
arguments.

A connection may be established in several ways:

provisioned MID

The MG may explicitely invoke megaco:connect/4 and use
a provisioned MID of the MGC as the RemoteMid.

upgrade preliminary MID

The MG may explicitely invoke megaco:connect/4 with the
atom 'preliminary_mid' as a temporary MID of the MGC,
send an intial message, the Service Change Request, to
the MGC and then wait for an initial message, the
Service Change Reply. When the reply arrives, the Megaco
application will pick the MID of the MGC from the
message header and automatically upgrade the connection
to be a "normal" connection. By using this method of
establishing the connection, the callback function
UserMod:handle_connect/2 to be invoked twice. First with
a ConnHandle with the remote_mid-field set to
preliminary_mid, and then when the connection upgrade is
done with the remote_mid-field set to the actual MID of
the MGC.

automatic

When the MGC receives its first message, the Service
Change Request, the Megaco application will
automatically establish the connection by using the MG
MID found in the message header as remote mid.

distributed

When a user (MG/MGC) is distributed over several nodes,
it is required that the node hosting the connection
already has activated the connection and that it is
in the "normal" state. The RemoteMid must be a real
Megaco MID and not a preliminary_mid.

An initial megaco_receive_handle record may be obtained
with megaco:user_info(UserMid, receive_handle)

The send handle is provided by the preferred transport
module, e.g. megaco_tcp, megaco_udp. Read the documentation
about each transport module about the details.

The connect is done in two steps: first an internal
connection setup and then by calling the user
handle_connect
callback function. The first step could result in
an error with Reason = connect_reason() and the second
an error with Reason = handle_connect_reason():

connect_reason()

An error with this reason is generated by the
megaco application itself.

handle_connect_reason()

An error with this reason is caused by the user
handle_connect
callback function either returning an error
or an invalid value.

When sending one transaction in a message, Actions should be
action_reqs() (UserReply will then be
user_reply()). When sending several transactions in a message,
Actions should be [action_reqs()] (UserReply
will then be [user_reply()]). Each element of the list is
part of one transaction.

For some of our codecs (not binary), it is also possible
to pre-encode the actions, in which case Actions will be
either a binary() or [binary()].

The function returns when the reply arrives, when the
request timer eventually times out or when the outstanding
requests are explicitly cancelled.

The default values of the send options are obtained by
megaco:conn_info(ConnHandle, Item). But the send options
above, may explicitly be overridden.

The ProtocolVersion version is the version actually encoded
in the reply message.

A message_error(), indicates that the remote user has
replied with an explicit transactionError.

A user_cancel_error(), indicates that the request has been
canceled by the user. reason_for_user_cancel() is the reason
given in the call to the cancel
function.

A send_error(), indicates that the send function of the
megaco transport callback module failed to send the request.
There are two separate cases: send_cancelled_reason() and
send_failed_reason().
The first is the result of the send function returning
{cancel, Reason} and the second is some other kind of
erroneous return value. See the
send_message
function for more info.

An other_error(), indicates some other error such as
timeout.

For more info abount the extra() part of the
result, see the
note
in the user callback module documentation.

Sends one or more transaction request(s) but does NOT wait for a reply

When sending one transaction in a message, Actions should be
action_reqs(). When sending several transactions in a message,
Actions should be [action_reqs()]. Each element of the
list is part of one transaction.

For some of our codecs (not binary), it is also possible
to pre-encode the actions, in which case Actions will be
either a binary() or [binary()].

The default values of the send options are obtained by
megaco:conn_info(ConnHandle, Item). But the send options above,
may explicitly be overridden.

The ProtocolVersion version is the version actually encoded
in the reply message.

The callback function UserMod:handle_trans_reply/4 is invoked
when the reply arrives, when the request timer eventually
times out or when the outstanding requests are explicitly
cancelled. See the megaco_user module for more info about
the callback arguments.

Encodes lists of action requests for one or more transaction
request(s).

When encoding action requests for one transaction,
Actions should be action_reqs().
When encoding action requests for several transactions,
Actions should be [action_reqs()]. Each element
of the list is part of one transaction.

This causes outstanding megaco:call/3 requests to return.
The callback functions UserMod:handle_reply/4 and
UserMod:handle_trans_ack/4 are also invoked where it
applies. See the megaco_user module for more info about the
callback arguments.

This function is intended to be invoked by some
transport modules when get an incoming message. Which
transport that actually is used is up to the user to
choose.

The message is delivered as an Erlang binary and is decoded
by the encoding module stated in the receive handle together
with its encoding config (also in the receive
handle). Depending of the outcome of the decoding various
callback functions will be invoked. See megaco_user for more
info about the callback arguments.

The argument Extra is just an opaque data structure passed to the user
via the callback functions in the
user callback module.
Note however that if Extra has the value
extra_undefined the argument will be ignored (same as if
process_received_message/4 had been called).
See the documentation for the behaviour of the callback module,
megaco_user, for more info.

Note that all processing is done in the context of the calling
process. A transport module could call this function via one of the
spawn functions (e.g. spawn_opt). See also
receive_message/4,5.

If the message cannot be decoded the following callback
function will be invoked:

UserMod:handle_syntax_error/3

If the decoded message instead of transactions contains a
message error, the following callback function will be
invoked:

UserMod:handle_message_error/3

If the decoded message happens to be received before the
connection is established, a new "virtual" connection is
established. This is typically the case for the Media
Gateway Controller (MGC) upon the first Service Change.
When this occurs the following callback function will be
invoked:

UserMod:handle_connect/2

For each transaction request in the decoded message the
following callback function will be invoked:

UserMod:handle_trans_request/3

For each transaction reply in the decoded message the reply
is returned to the user. Either the originating function
megaco:call/3 will return. Or in case the originating
function was megaco:case/3 the following callback function
will be invoked:

UserMod:handle_trans_reply/4

When a transaction acknowledgement is received it is
possible that user has decided not to bother about the
acknowledgement. But in case the return value from
UserMod:handle_trans_request/3 indicates that the
acknowledgement is important the following callback function
will be invoked:

UserMod:handle_trans_ack/4

See the megaco_user module for more info about the callback
arguments.

When evaluating a digit map, a state machine waits for
timeouts and letters reported by
megaco:report_digit_event/2. The length of the various
timeouts are defined in the digit_map_value() record.

When a complete sequence of valid events has been received,
the result is returned as a list of letters.

There are two options for handling syntax errors (that is
when an unexpected event is received when the digit map
evaluator is expecting some other event). The unexpected
events may either be ignored or rejected. The latter means
that the evaluation is aborted and an error is returned.

When decoding property_group() or
property_groups(),
those property parameter constructs that cannot be decoded
(either because of decode error or because they are unknown),
will be returned as a two-tuple. The first element of which
will be the (undecoded) property parameter and the other the
actual reason.
This means that the caller of this function has to expect not
only sdp-records, but also this two-tuple construct.

Retreive the (SNMP) statistic counters maintained by the
megaco application. The global
counters handle events that cannot be attributed to
a single connection (e.g. protocol errors that occur
before the connection has been properly setup).

This function is only intended for testing purposes. It's
supposed to have a same kind of interface as the call or cast functions (with the additions
of the EncodingMod and EncodingConfig
arguments). It composes a complete megaco message end
attempts to encode it. The return value, will be a tuple of
the composed megaco message and the encode result.

This function is only intended for testing purposes. It's
supposed to test the actual_reply() return value of
the callback functions
handle_trans_request
and
handle_trans_long_request
functions (with the additions of the EncodingMod and
EncodingConfig arguments). It composes a complete
megaco message end attempts to encode it. The return value,
will be a tuple of the composed megaco message and the
encode result.